Wastewater Treatment

Clean water is a basic need for all human beings, which is used for drinking, cooking, washing, and several other purposes. Population growth, industrial growth, pollution, and mismanagement of resources have led to the scarcity of usable water.

Water gets dirty and waste-laden due to usage, and is known as sewage. This sewage or wastewater must be treated properly so that it can be reused as clean water.

Stages of Waste Water Processing

Screening

Large items from the influent are removed to prevent damage to the equipments of the treatment plant.

Grit Removal

Influent flows through a grit chamber where the grit is removed.
Grit chambers (horizontal, aerated, or vortex) control the flow of water, and the heavier grit is allowed to sink to the bottom of the chamber. It is physically removed from the bottom of the chamber and discarded.
Water and organic material continue to flow to the next stage.

Primary Clarifier

The influent enters primary clarifiers that separate some of the solid organic matter from the influent.
The large clarifiers allow the heavy solids known as ‘sludge’ to sink to the bottom and the cleaner influent to flow.
Only a portion of dried sludge is used as fertiliser or soil conditioner; excess may be incinerated or sent to landfills.
Appropriate water flow controls the effectiveness of the primary clarifier.
If the water flow is too fast, the solids do not sink and settle properly, and if the water flow is too slow, it affects the process upstream.

Aeration

During aeration, oxygen is pumped into the tank. It is accomplished by either defusing air into the tank or through agitation that adds air to the water.
It provides the best conditions for bacterial growth.
Dissolved oxygen is critically monitored since low oxygen (<2 ppm) does not kill all bacteria, but reduces the efficiency of aerobic bacteria; anaerobes may survive.
The efficiency of the bacteria in converting ammonia (NH₃) to nitrogen (N₂) is measured. Conversion pathway is ammonia (NH₃) → nitrite (NO₂⁻) → nitrate (NO₃⁻) → nitrogen gas (N₂) through nitrification and denitrification.
BOD (Biological Oxygen Demand) indicates the amount of organic material present in the effluent, and thus determines the effectiveness of organic material breakdown.

Secondary Clarifier

The influent exits the aeration tank and flows into a secondary clarifier.
Similar to the primary clarifier, fine solids sink to the bottom of the tank.
These small solids are the ‘activated sludge’ and consist of active bacteria.
A portion of this activated sludge returns to the aeration tank to increase the bacterial population and accelerate the breakdown of organic material, and the excess is discarded.

Chlorination

The effluent is tested for the absence or presence of bacteria due to the treatment in the aeration tank. It ensures that pathogenic microorganisms are destroyed.
Chlorination is an inexpensive type of disinfection that kills any remaining bacteria in the contact chamber. Nowadays, ozone and UV are also used for disinfection.

Water Analysis

Proper levels of pH, dissolved oxygen, ammonia, nitrates, and residual chlorine are tested.
The final testing is done to confirm the status of the effluent (clean water) leaving the treatment plant.

Finally, the clean water is reintroduced into the environment.

Frequently Asked Questions (FAQs)

Q1. What are flocs?

A. Flocs are mesh-like structures consisting of aerobic bacteria and fungal filaments. During aeration, the sewage is pumped into large tanks where the flocs consume the organic matter present in the wastewater and reduce the pollution level. Later on, these flocs settle down as activated sludge.

Q2. How is biogas produced during wastewater treatment?

A. A huge amount of the activated sludge is processed in anaerobic sludge digesters, which are large, oxygen-free tanks. The bacteria present within the digesters break down the organic matter in the sludge to release biogas, which is actually a mixture of gases primarily consisting of methane, carbon dioxide, and hydrogen sulphide. Biogas is combustible and can be used to generate electricity in the treatment plant, making the entire process more sustainable.

Q3. Why is grit removed separately from sewage during wastewater treatment?

A. Grit (sand, gravel, small stones) can damage pumps and machinery, so it is removed early in the process.

Q4. What is the role of activated sludge in wastewater treatment?

A. Activated sludge contains aerobic microbes that break down organic matter, making the water cleaner and reducing BOD.